The ear is part of a sensory system found in most carbon-based lifeforms, typically, but not always, consisting of a fleshy outer appendage usually called the outer ear, as well as a group of canals and vibration-sensitive membranes and small bones located inside the skull. Ears function by receiving vibrations in the ambient atmosphere and translating them into electrical impulses sent to the brain, which allows a humanoid to 'hear' sounds around them.

Human ears are typically mostly round, fleshy appendages, while Vulcan ears are more typically pointed at the top. These features made Vulcans the frequent subjects of ridicule by Humans, although the stoic Vulcans were confounded by Human "fascination" with their ears.

Many species, such as the Gorn have no external appendage or visible 'ear' but can hear as well as, and in some cases far better than, species with external auditory organs. (ENT: "In a Mirror, Darkly, Part II")

The sensitivity and hearing ranges for species vary widely, even on the same planet. Vulcan hearing is known to be more acute than that of most Humans. Ferengi have extremely sensitive hearing, as well. Vorta were genetically engineered with no aesthetic sense, but acute hearing, as Weyoun was able to hear GulDukat's quiet discussion about his daughter with his subordinate Damar. (DS9: "Favor the Bold")

In "Distant Voices", "Elim Garak", in Bashir's mind, said Cardassian hearing is not as acute as Humans, though he later turned out to be Altovar, another character in Bashir's mind, so this may not be true.

The state of being unable to hear, whether by genetic makeup, congenital defect, or external damage to a being's hearing is known as deafness. Some societies consider deafness a weakness or handicap, while others have no social stigma attached to it. (TNG: "Loud As A Whisper")